A liquid crystal optical element for optical modulation, particularly for light phase modulation, which is different from a normal liquid crystal element for display, has been used for correcting aberration in an optical pickup device for a DVD or the like. This liquid crystal optical element optically corrects wave aberration (mainly coma) that is generated in a disk substrate of a DVD due to the inclination of the optical axis of a laser beam irradiated onto the DVD.
FIG. 9A and FIG. 9B are views showing the concept of a structure of a liquid crystal optical element 20 that is used in a conventional optical pickup device. FIG. 9B is a cross-sectional view cut along the line A–A′ in FIG. 9A. The liquid crystal optical element 20 is constructed of two transparent substrates, each formed with a transparent electrode and an orientation film, that are adhered together with a sealing agent, leaving a predetermined clearance between the two transparent substrates. Liquid crystal is sealed in this clearance. As shown in FIG. 9A and FIG. 9B, the liquid crystal optical element 20 is formed with a light-receiving surface for making a laser beam incident thereto. Four sides of this surface are surrounded by a sealing member 1.
A first transparent substrate 7 of the liquid crystal optical element 20 is formed with a plain electrode 8 as a transparent opposite electrode, and an alignment film 9. A second transparent substrate 11 of the liquid crystal optical element 20 is formed with a transparent electrode 4 for correcting aberration, a transparent wiring pattern 5 for the transparent electrode 4, and an alignment film 10. The first transparent substrate 7 and the second transparent substrate 11 are adhered to each other with the sealing member 1, with a predetermined clearance left between the transparent substrates. A liquid crystal 6 is sealed in this clearance. The first and second transparent substrates are held with the sealing member. In actual practice, various patterns for correcting aberration, not shown in the drawings, are formed on the transparent electrode 4 for correcting aberration.
An effective diameter 2 of a laser beam incident to the liquid crystal optical element 20 is shown in FIG. 9A. The “effective diameter” will hereinafter refer to a main beam diameter that can be utilized effectively by an objective lens (for example, an objective lens 15 shown in FIG. 10) on the liquid crystal optical element on geometrical optical design, involving no positional deviation or diameter change in the laser beam.
In recent years, there has been strong demand for reducing the size of the optical pickup device for a DVD. To meet this demand, it is also necessary to reduce the size of the liquid crystal optical element 20. However, as the liquid crystal optical element 20 has the four sides of the light-receiving surface surrounded by the sealing member 1, this sealing member 1 occupies some area. In order to reduce the size of the liquid crystal optical element 20, it is has been considered to design and manufacture it by setting the light-receiving surface to have a size as close as possible to the effective diameter 2, thereby to reduce the area of the sealing member 1.
However, there is a drawback in that the performance of the liquid crystal optical element is lowered when the size of the light-receiving surface is set close to the size of the effective diameter 2. The main reason for this is that impurity and uncured resin components from the sealing member 1 affect the liquid crystal 6 or the alignment films 9 and 10, where the sealing member 1 is usually made of resin and the alignment films 9 and 10 are positioned near the sealing member 1. Therefore, in order to avoid the above problem, it is necessary to design and manufacture the light-receiving surface to have a size sufficiently larger than the effective diameter 2.
Further, in order to drive the aberration-correcting transparent electrode 4 disposed on the light-receiving surface, it is necessary to arrange the transparent wiring pattern 5 near the electrode 4. However, this has a further drawback, described below, on the transparent wiring pattern 5 and the area between the transparent electrode 4 and the transparent wiring pattern 5.
FIG. 10 shows one example of an optical pickup device that uses the conventional liquid crystal optical element 20. As shown in FIG. 10, a laser beam emitted from a laser beam source 12 is changed into a parallel beam by a collimator lens 13, the diameter of the parallel beam being regulated by a diaphragm 14. The beam passes through the liquid crystal optical element 20, and is irradiated to a DVD 16 by an objective lens 15.
The liquid crystal optical element 20 is provided with the transparent electrode 4 for correcting aberration, in order to cover the range of the effective diameter 2 that is designed in advance. However, the effective diameter 2 is a value obtained based on a geometrical optical calculation, and an actual laser beam has a wave optical diffraction spread. Therefore, a luminous flux 17 of a laser beam that is incident to the liquid crystal optical element 20 becomes larger than the effective diameter 2. The laser beam has a foot component, though at a low level, at the outside of the effective diameter 2.
The liquid crystal optical element 20 has an area where the transparent wiring pattern 5 is provided, at the outside of the transparent electrode 4, as described above. Also, the area is disposed on the liquid crystal 6 sealed within the sealing member 1. Further, the transparent electrode 4 for correcting aberration has a edge.
When the foot component of a laser beam is irradiated to this area, the beam is diffracted and scattered to generate a normal diffraction beam and a normal scattering beam 18, based on the edge of the transparent electrode 4 and the transparent wiring pattern 5. At the time of driving the transparent electrode 4 for correcting aberration, a current is supplied to the transparent wiring pattern 5, and the liquid crystal 6 is driven between the transparent electrode 4 and the transparent opposite electrode 8. When the foot component of the laser beam is irradiated to the liquid crystal 6, a modulation beam 19 is generated.
The normal diffraction beam and the normal scattering beam 18 interfere with the laser beam for irradiating the DVD, and this degrades the laser beam. As the laser beam has high coherence, the diffraction beam and the scattering beam cause a bad influence to the optical system. The diffraction beam and the scattering beam become a noise beam, which lowers the intensity of the effective transmission light. According to the results of measurement carried out by the present inventor, the diffraction beam and the scattering beam became a noise beam, and this lowered the intensity of the effective transmission light by five percent.
The modulation beam 19 causes the intensity of the laser beam for irradiating the DVD to modulate. In particular, at the time of writing to the DVD in a DVD-R system, it is necessary to keep the intensity of the laser beam constant. Therefore, the modulation beam 19 is the trouble to be solved.
For the above reasons, according to the conventional liquid crystal optical element, it has been necessary to dispose the transparent wiring pattern 5 on an outer portion separated sufficiently from the area to which the foot component of the laser beam is irradiated. Consequently, the liquid crystal optical element 20 has had to have a larger size, thereby hindering reduction in the size of the optical pickup device.
The diaphragm 14 disposed in the optical path of the laser beam between the laser beam source 12 and the liquid crystal optical element has not been able to sufficiently shield the foot component of the laser beam. Viewed at the wave optics, the laser beam that has passed through the diaphragm 14 is propagated to the liquid crystal optical element 20 while expanding its diameter due to the diffraction between the laser beam and the diaphragm 14.